It is important to chase the function of islet after transplantation. Thus, we examined the correlation between grafted islets function and molecular imaging intensity using Cy5.5 labeled islet. The ability of Cy5.5-PEG-NHS to chemically bind on the surface of islets was determined by confocal laser scanning microscope. Then, fluorescence intensity of different number of Cy5.5 labeled islets was determined using optical imaging system. We have found out the intensity of fluorescence increased with increasing the total number of islet in each well. In addition, different number of Cy5.5-labeled islet has been transplanted into the athymic mice for in vivo imaging. The intensity emitted from Cy5.5-labeled islets augmented proportionally with increased number of transplanted islets. To understand the correlation between the function of grafted islet and the fluorescence intensity emitted optical imaging system, Cy5.5-labeled islets have been transplanted into F344 rats. The results revealed that there was a correlation between the fluorescence intensity and the non-fasting blood glucose (NBG) levels of islets received rats. Strong fluorescence intensity corresponded to low NBG whereas low signal was associated to high NBG. In conclusion, the fluorescence intensity emitted from Cy5.5-labeled islets can be used as a marker of cells viability and functionality after transplantation.